The present invention relates to a process for producing a metal vacuum bottle by a vacuum brazing operation, and in particular to a process which provides a metal vacuum bottle having an enhanced degree of vacuum within a thermal insulation layer portion thereof.
In general, a metal vacuum bottle has been produced in such a manner that after the internal and the external shells having been seamed together at their top openings, an evacuation of a jacket between said two shells is carried out through the vacuum suction port which is beforehand provided on the bottom part of the external shell. However, the vacuum thermal insulation effect is often difficult to maintain for a long period of time because of leakage from the inner peripheral surface of the external shell and the outer peripheral surface of the internal shell or other parts. In order to maintain the desired thermal insulation effect, it is necessary to maintain the high degree of vacuum by repeating a proper evacuating process, but such operation for repeating evacuation as set forth above is very complicated as well as possibly deteriorating the value of merchandise.
From the aforesaid shortcomings, a process for producing a metal vacuum bottle of the type mentioned above by a vacuum brazing operation has been proposed in which a substantially complete degassing treatment can be carried out by subjecting the peripheral wall surfaces of said internal and external shells to a heating treatment and evacuating a jacket between said two shells. Furthermore, production is designed to be simplified. Said process comprises the steps of transferring the assembly of a metal vacuum bottle to a vacuum heating furnace for heating and evacuating gases from said assembly which is previously set with a filler metal at the seamed portion between the internal and the external shells, and of brazing said seamed portion by heating at a temperature higher than the melting point of the filler metal under the desired degree of vacuum, for instance, lower than 10.sup.-2 Torr. According to the above-mentioned process, the assembly is subjected to vacuum baking when the internal and the external shells of a vacuum bottle are brazed together, so that the degassing treatment for said peripheral surfaces is effected at the same time, thereby favorably avoiding deterioration of the vacuum thermal insulation effect in the course of aging. In a conventional process, two steps have generally been necessary for producing a vacuum bottle, one of which is brazing the internal and the external shells together and the other step being evacuation of space between said two shells. In the foregoing proposed process, however, the aforesaid steps can advantageously be carried out concurrently. Unfortunately, many troubles and inconveniences have occurred in putting said proposed process to a practical use. For instance, it is imperative to obtain a sufficiently high degree of vacuum because of the faculty of a vacuum bottle being dependent upon the thermal insulation effect. It is desirable to provide said high degree of vacuum with all possible haste. However, when the vacuum brazing operation is applied to the process, the difference between the vacuum furnace and the thermal insulation layer portion of a vacuum bottle in view of the vacuum degree is brought forth due to influence of conductance, so that the evacuation should be carried on for a long period of time in order to acquire a desired vacuum degree. Said troubles are caused by the fact that it is not desirable to enlarge the sectional area of the vacuum suction port formed on the thermal insulation layer portion. (Said suction port is heated to be sealed by a filler metal at the melting temperature thereof). On the other hand, if the vacuum suction port is enlarged in anticipation of the evacuation effect, the operation for sealing with a brazing filler metal becomes complicated and may cause an unreliable sealing. Furthermore, in many cases the desired level of the vacuum after subjecting the vacuum suction port to the vacuum brazing operation may not be maintained owing to residual gas within the thermal insulation layer portion, said gas having been generated at the time of the brazing filler metal being melted. Still further, in the vacuum brazing operation for producing a vacuum bottle, the whole body of said vacuum bottle has to be heated at a high temperature so that, in case of producing a vacuum bottle with austenite stainless steel or the like, it should be influenced by the annealing effect which causes a problem in that the external shell will have to be of substantial thickness. An increase in weight results as well as a considerable rise in the cost of production, etc.